This invention relates to nickel-base superalloys used in high-temperature applications, and, more particularly, to articles made of such materials and having a thermal barrier coating with a platinum-aluminide bond coat.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys and can operate at temperatures of up to 1900-2100xc2x0 F.
Many approaches have been used to increase the operating temperature limit of the turbine blades and vanes. The compositions and processing of the materials themselves have been improved. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cool air is forced through the channels during engine operation.
In another approach, a metallic protective coating or a ceramic/metal thermal barrier coating system is applied to the turbine blade or turbine vane component, which acts as a substrate. The metallic protective coating is useful in intermediate-temperature applications. One known type of metallic protective coating is a platinum-aluminide coating that is formed by first depositing platinum and then aluminum onto the surface of the substrate, and then interdiffusing these constituents. A ceramic thermal barrier coating may be applied overlying the platinum-aluminide coating.
The thermal barrier coating system is useful in high-temperature applications. The ceramic thermal barrier coating insulates the component from the exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component. However, ceramic layers usually do not adhere well directly to the nickel-base superalloys used in the substrates. Therefore, the platinum aluminide bond coat is placed between the substrate and the thermal barrier coating to effect the adhesion of the ceramic layer to the substrate. In addition, the upper surface of the bond coat oxidizes to a protective aluminum oxide scale to inhibit oxidation of the substrate.
While superalloys coated with such ceramic/metal thermal barrier coating systems do provide substantially improved performance over uncoated materials, there remains opportunity for improvement in elevated temperature performance and environmental resistance. There is an ongoing need for improved bond coats to protect nickel-base superalloys in elevated temperature applications. This need has become more acute with the development of the newest generation of nickel-base superalloys, inasmuch as the older protective coatings are often not satisfactory with these materials. The present invention fulfills this need, and further provides related advantages.
The present invention provides a metallic coating for nickel-base superalloys. The overcoating comprises a platinum-aluminide layer useful as a metallic protective coating or as a bond coat for a thermal barrier coating system. The overcoating is in the form of a surface region that is well bonded to the substrate. The platinum-aluminide coating has good elevated-temperature stability and resistance to environmental degradation in typical gas-turbine engine applications.
In accordance with the invention, a method for preparing an article having a substrate protected by an overlying coating comprises the steps of furnishing a substrate comprising a nickel-base superalloy, thereafter depositing a first layer comprising platinum contacting an upper surface of the substrate, and thereafter depositing a second layer comprising aluminum contacting an upper surface of the first layer, leaving an exposed surface. The method further includes final desulfurizing the article to yield an article with a final-desulfurized exposed surface. The step of final desulfurizing includes the steps of heating the article to a final desulfurizing elevated temperature, and thereafter removing material from the exposed surface of the article. The heating is preferably accomplished in a reducing gas such as hydrogen, but useful results may also be achieved by heating in vacuum or even in air in some cases. After the step of final desulfurizing, a ceramic coating may be deposited overlying the final desulfurized second layer.
Optionally but desirably, after the step of depositing a first layer and before the step of depositing a second layer, the article is intermediate desulfurized at least once by heating the article to an elevated temperature in a reducing gas. After this intermediate desulfurizing step, it is preferred to remove at least some material from the exposed surface.
The deposition of the platinum-containing first layer and the aluminum-containing second layer leave these layers with a relatively high sulfur content. In particular, the first layer is normally electrodeposited onto the surface of the substrate, which leaves a high sulfur content in the first layer. The presence of excess sulfur encourages debonding of aluminum oxide formed on the surface of the bond coat but below the ceramic coating, and thence debonding of the ceramic coating, from the surface of the substrate during subsequent service. The present approach reduces the sulfur content of the platinum-aluminide interdiffused layer to a low level, about 10 parts per million by weight (ppmw) or less. The result is improved adherence of the aluminum oxide and the ceramic top coat during service.
Each heating step causes sulfur to diffuse from the bulk interior of the coating layer nearest the surface, toward the exposed surface. If the heating is performed in hydrogen, some of the sulfur reaching the exposed surface reacts with the hydrogen and is removed as hydrogen sulfide gas. However, it has been observed that chemical effects (e.g., interaction with yttrium, calcium, and possibly other minor elements present in the alloy substrate and/or the bond coat layers) and the presence of oxide layers at the exposed surface inhibit some of the sulfur from combining with hydrogen and leaving the surface, in at least some systems. The removal of some material at the exposed surface after the heating, such as by grit blasting or honing, serves to remove some of this region of excess sulfur. The newly exposed surface after the material removal has a lower sulfur content which does not encourage debonding of the subsequently formed aluminum oxide. The amount of material removed is typically quite small, on the order of from about 0.5 to about 2 micrometers, and for most applications this amount of material removal is negligible in relation to the dimensions of the final product. However, the layers may be deposited slightly thicker than otherwise would be the case, to account for the material removed.
The present approach results in a substantially reduced sulfur content in the platinum-aluminide coating, and thence in the final coated structure, with the reduction being achieved at the surface where the next overlying layer is thereafter deposited. The bond coat and the ceramic top layer achieve better adhesion during service as a result. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.